1. Field of the Invention
The present invention relates generally to stereolithography and, more specifically, to the use of stereolithography in the packaging of electronic components. Use of a machine vision system such as a pattern recognition system to facilitate application of stereolithographic techniques to fabrication of electronic components and other products is encompassed in the invention.
2. State of the Art
In the past decade, a manufacturing technique termed “stereolithography,” also known as “layered manufacturing,” has evolved to a degree where it is employed in many industries.
Essentially, stereolithography, as conventionally practiced, involves utilizing a computer to generate a three-dimensional (3-D) mathematical simulation or model of an object to be fabricated, such generation usually effected with 3-D computer-aided design (CAD) software. The model or simulation is mathematically separated or “sliced” into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object, and surface resolution of the object is, in part, dependent upon the thickness of the layers.
The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by different companies has resulted in techniques for fabrication of objects from both metallic and non-metallic materials. Regardless of the material employed to fabricate an object, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries, followed by selective consolidation or fixation of the material to at least a semi-solid state in those areas of a given layer corresponding to portions of the object, the consolidated or fixed material also at that time being substantially concurrently bonded to a lower layer. The unconsolidated material employed to build an object may be supplied in particulate or liquid form, and the material itself may be consolidated or fixed or a separate binder material may be employed to bond material particles to one another and to those of a previously formed layer. In some instances, thin sheets of material may be superimposed to build an object, each sheet being fixed to a next lower sheet and unwanted portions of each sheet removed, a stack of such sheets defining the completed object. When particulate materials are employed, resolution of object surfaces is highly dependent upon particle size, whereas when a liquid is employed, surface resolution is highly dependent upon the minimum surface area of the liquid which can be fixed and the minimum thickness of a layer which can be generated. Of course, in either case, resolution and accuracy of object reproduction from the CAD file is also dependent upon the ability of the apparatus used to fix the material to precisely track the mathematical instructions indicating solid areas and boundaries for each layer of material. Toward that end, and depending upon the layer being fixed, various fixation approaches have been employed, including particle bombardment (electron beams), disposing a binder or other fixative (such as by ink-jet printing techniques), or irradiation using heat or specific wavelength ranges.
An early application of stereolithography was to enable rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed might be rapidly generated. Prototypes of objects might be built to verify the accuracy of the CAD file defining the object and to detect any design deficiencies and possible fabrication problems before a design was committed to large-scale production.
In more recent years, stereolithography has been employed to develop and refine object designs in relatively inexpensive materials, and has also been used to fabricate small quantities of objects where the cost of conventional fabrication techniques is prohibitive for same, such as in the case of plastic objects conventionally formed by injection molding. It is also known to employ stereolithography in the custom fabrication of products generally built in small quantities or where a product design is rendered only once. Finally, it has been appreciated in some industries that stereolithography provides a capability to fabricate products, such as those including closed interior chambers or convoluted passageways, which cannot be fabricated satisfactorily using conventional manufacturing techniques.
However, to the inventors' knowledge, stereolithography has yet to be applied to mass production of articles in volumes of thousands or millions, or employed to produce, augment or enhance products including other, pre-existing components in large quantities, where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results is required. Furthermore, conventional stereolithography apparatus and methods fail to address the difficulties of precisely locating and orienting a number of pre-existing components for stereolithographic application of material thereto without the use of mechanical alignment techniques or to -otherwise assuring precise, repeatable placement of components.
In the electronics industry, state-of-the-art packaging of semiconductor dice is an extremely capital-intensive proposition. In many cases, semiconductor dice carried on, and electrically connected to, lead frames are individually packaged with a filled-polymer material in a transfer molding process. A transfer molding apparatus is extremely expensive, costing at least hundreds of thousands of dollars in addition to the multi-hundred thousand dollar cost of the actual transfer molding dies in which strips of lead frames bearing semiconductor dice are disposed for encapsulation.
So that the reader may more fully understand the present invention in the context of the prior art, it seems appropriate to provide a brief description of a transfer apparatus and method for forming a plastic package about an LOC die assembly. The term “transfer” molding is descriptive of this process as the molding compound, once melted, is transferred under pressure to a plurality of remotely-located mold cavities containing die assemblies to be encapsulated.
FIG. 9 is a flow chart of a typical process sequence for plastic package molding. It should be noted that the solder dip/plate operation has been shown as one step for brevity; normally, plating would occur prior to trim and form. FIGS. 10A and 10B show pre-molding and post-molding positions of encapsulant during a transfer molding operation using a typical mold apparatus comprising upper and lower mold halves 410 and 412, each mold half including a platen 414 or 416 with its associated chase 418 or 420. Heating elements 422 are employed in the platens to maintain an elevated and relatively uniform temperature in the runners and mold cavities during the molding operation. FIG. 11 shows a top view of one side of the transfer mold apparatus of FIGS. 10A and 10B.
In operation, a heated pellet of resin mold compound 430 is disposed beneath ram or plunger 432 in pot 434. The plunger descends, melting the pellet and forcing the melted encapsulant down through sprue 436 and into primary runner 438, from which it travels to transversely-oriented secondary runners 440 and across gates 442 into and through the mold cavities 444, wherein die assemblies 500 comprising dies 100 with attached lead frames 502 are disposed (usually in strips so that a strip of six lead frames, for example, would be cut and placed in and across the six cavities 444 shown in FIG. 11). Air in the runners 438 and 440 and mold cavities 444 is vented to the atmosphere through vents 446 and 448. At the end of the molding operation, the encapsulant is “packed” by application of a higher pressure to eliminate voids and reduce nonuniformities of the encapsulant in the mold cavities 444. After molding, the encapsulated die assemblies are ejected from the cavities 444 by ejector pins 450, after which they are post-cured at an elevated temperature to complete cross-linking of the resin mold compound 430, followed by other operations as known in the art and set forth in FIG. 1 by way of example. It will be appreciated that other transfer molding apparatus configurations, as well as variations in the details of the described method are known in the art. However, none of such are pertinent to the invention, and so will not be discussed herein.
Encapsulant flow in the mold cavities 444 is demonstrably non-uniform. The presence of the die assembly 500 comprising a die 100 with lead frame 502 disposed across the mid-section of a cavity 444 splits the viscous encapsulant flow front into upper and lower components. Further, the presence of the (relatively) large die 100 with its relatively lower temperature in the middle of a cavity 444 permits the flow front on each side of the die 100 to advance ahead of the front which passes over and under the die 100.
Encapsulant filler particles may become lodged between lead ends and the underlying die surfaces. The non-uniform flow characteristics of the viscous encapsulant flow may cause particles to be more forcefully driven between the lead ends and the die 100 and wedged or jammed in place in low-clearance areas. As the encapsulant flow front advances and the mold operation is completed by packing the cavities, pressure in substantially all portions of the mold cavities reaches hydrostatic. With LOC arrangements where lead ends extending over the active surface of a die 100 are bonded thereto by adhesive-coated tape or an adhesive material patterned on the active surface, the relative inflexibility of the tightly-constrained (adhered) lead ends maintains the point stresses of any particles trapped under the lead ends. These residual stresses are carried forward in the fabrication process to post-cure and beyond. When mechanical, thermal or electrical stresses attendant to post-encapsulation processing are added to the residual point stresses associated with the lodged filler particles, cracking or perforation of the die coat may occur, with the adverse effects previously noted. It has been observed that filler particle-induced damage occurs more frequently in close proximity to the adhesive, where lead flexure potential is at its minimum. In addition to damage by filler particles, transfer molding also results in the problem of bond wire sweep, wherein bond wires may be damaged, broken, loosened from their connections to bond pads or lead ends or swept into shorting contact with an adjacent bond wire under the impetus of the flow front of molten resin encapsulant as it flows through a mold cavity.
In addition to end-product deficiencies as noted above due to the phenomena of particulate die coat penetration and bond wire sweep, the capital-intensive nature of the transfer molding apparatus, including the requirement for different, multi-hundred thousand dollar molds for each die and lead frame arrangement as well as the high cost of the encapsulant resin and waste of same which is not used in the mold cavities, renders the transfer molding process an extremely expensive one. Mold damage and refurbishment is an additional, ongoing cost. Further, the elevated temperatures used in the molding process as well as in the post cure of the resin encapsulant is detrimental to the circuitry of the die as well as to the electrical connections to the lead ends.